1. Field of the Invention
The present invention relates to apparatus and methods for charging rechargeable batteries. More specifically, the present invention relates to apparatus and method for charging, discharging and recharging rechargeable batteries under adverse thermal conditions.
2. Description of the Related Art
Reliable electric power sources are needed to meet the continued growth of electric and electronic business, commercial and personal applications. For portable applications, the chemical storage battery is most commonly employed. For fixed location applications, the public power grid is the most common source of electrical power. Also, alternative sources of power are often employed to produce electric power, such as solar-voltaic, thermal, wind, water and other power sources.
For many applications, a high degree of reliability is required. Although public power grids are highly reliable, these grids are not perfect. Nor are alternative sources of electric power. Therefore, storage batteries are frequently employed in conjunction with, and as a back-up to, the public power grid and alternative sources of electrical power.
Chemical storage batteries have been produced using a variety of technologies. Each technology comprises a number of defining characteristics that should be considered in selecting a suitable technology for a particular application. These include, but are not limited to, size, weight, cost, power density, environmental constraints, voltage, current, power, and so forth.
In many applications, the ability to be recharged is a critical requirement of a chemical storage battery. Rechargeability reduces cost, extends useful life, and adds reliability to both battery and system design. Some common chemical technologies employed in rechargeable batteries are Nickel-Metal Hydride, Lithium Ion, Lithium Ion Polymer, Lead-Acid, and Nickel Cadmium among other unique and hybrid technologies.
Rechargeable batteries are charged by delivering electric current to positive and negative terminals of the battery for a duration of time sufficient to fully charge the battery. Later, current is drawn from the battery as a power source to some particular device or application.
However, the conditions of charging and discharging are not without limitations. The limitations are typically defined by the battery manufacturer or supplier. In applications where a battery is maintained as a back up to another primary source of electrical power, the battery may rest for long periods of time in a fully charged (xe2x80x9cstandbyxe2x80x9d) state, awaiting an interruption of the primary power source. When this occurs, the electric power stored in the battery is consumed in lieu of the primary power source.
A chemical battery resting in the standby state for long periods of time may degrade due to various factors. The total power available may be reduced, the terminal voltage may change, and the ability to determine the amount of power available may be compromised.
Smart battery charge algorithms have been developed to alleviate some of the problems associated with long term standby operation of a battery. Such chargers periodically xe2x80x98conditionxe2x80x99 the battery by applying an artificial load to discharge the battery to some predetermined level, and then recharge the battery to full charge. During such a conditioning process, certain metrics may be measured and used to calibrate the battery for later determination of the available power during a battery discharge cycle. It is desirable to process a discharge cycle in as short a period of time as possible so that the battery can quickly be returned to standby operation. Similarly, it is generally desirable to charge a battery as quickly as possible so that it can be readied for use as quickly as possible.
When a battery is being charged or discharged, a certain amount of internal heat is generated as current flows through the battery. This heat is proportional to the amount of current flowing within the battery. In ambient conditions where the amount of heat generated is small compared to the heat loss from the battery, the internal heat generation is usually not significant. Often, a battery is located in close physical proximity to the device it powers or to which it provides standby service. An example of this is occurs when a battery is used to provide standby power to a computing device. In most instances, the device with which the battery operates also generates heat during operation.
Electrical energy discharged from the battery can cause thermal problems at high temperature, for both the battery and the adjacent circuitry. For example, a battery may be subjected to heat energy produced by the device it powers as well as the heat the battery produces internally. In addition, the components adjacent to the battery conditioning circuit (often a resistive load) may be pushed close to thermal limits due to joule heating of the discharge load at high temperature.
In addition, other heat sources in the vicinity of the battery may affect ambient conditions and raise the operating temperature of the environment. Thus, it is not uncommon for a battery to be operated at substantially elevated temperatures.
When a battery is operating at or near its maximum operating temperature, designers are faced with a dilemma. If the battery charge and discharge currents are maintained at levels normally applied for the lower ranges of expected operating temperatures, the battery life and reliability can be greatly compromised when temperatures become elevated. On the other hand, if the designer takes a conservative approach, and sets the charge and discharge currents at levels consistent with a reasonable maximum operating temperature, then charge and discharge currents may be so low that the time required to accomplish these operations become unacceptably long.
Alternatives presently available to address this dilemma include locating the battery in a cooler environment, usually distant from the device being powered and providing additional cooling equipment. Each of these alternatives is typically undesirable due to increased cost, greater systems complexity, or reduced reliability, inter alia.
Thus there is a need in the art for an apparatus and method for efficiently charging, discharging and recharging batteries in environments with variable thermal conditions.
The need in the art is addressed by the apparatus and methods taught by the present invention. An apparatus for charging a battery according to its temperature is taught. The apparatus includes a charging circuit adapted to charge a battery and a temperature sensor positioned to sense a battery temperature, i.e., adjacent environmental temperature. The apparatus includes a controller coupled to the temperature sensor and the charging circuit. The controller operates to set the charging current in accordance with the sensed temperature.
In a refinement, the controller continuously sets the charging current in accordance with the sensed temperature. In a further refinement, the controller periodically sets the charging current in accordance the sensed temperature. In a further refinement, the apparatus further includes a memory coupled to the controller having a temperature and charging current look up table stored therein. In this embodiment, the controller accesses the look up table to set the charging current. In a further refinement, the controller operates to set the charging current to a maximum value when the temperature is lower than a first predetermined threshold value. In a further refinement, the maximum value is the battery""s maximum specified charging current and the first predetermined threshold value is the battery""s maximum charging temperature. In a further refinement, the controller sets the charging current to zero when the temperature is higher than a second predetermined threshold value. In a further refinement, the battery is coupled to a load and the temperature sensor senses the temperature of the battery and the load.
The present invention also teaches an apparatus for exercising or conditioning a battery. This apparatus includes the charging circuit and a temperature sensor. Also, a discharging circuit is coupled to the battery while a controller is coupled to the temperature sensor, the charging circuit, and the discharging circuit. The controller operates to set the charging and discharging currents in accordance with temperature.